The present invention relates to disc-shaped record carriers (optical discs) capable of optically recording and/or playing back information such as, for example, an optical data file, an optical video disc and an optical audio disc. More particularly, it relates to an optical disc which has a plurality of reference guide tracks recorded in advance and in which information can be recorded between the reference guide tracks.
Also, the present invention relates to an apparatus for recording information on the optical disc.
As an optical disc capable of recording and playback, there has heretofore been known one in which a recording film provided on a substrate is formed with minute pits by heating and vaporizing very small parts by means of a laser beam. In the recording of the optical disc of this type, when add-on recording is desired, it needs to be performed while a spacing from a track already recorded is held exactly constant. ln particular, there is a case where the disc is once taken off after several tracks have been recorded on a part of the disc and where the add-on recording is to be performed anew. 1n this case, the eccentric magnitude of the disc is usually as large as 100 .mu.m, and the track pitch is about 1.6 .mu.m. Therefore, a track add-on recorded anew crosses several tens of tracks already recorded, and accurate recording becomes impossible.
ln order to cope with this situation, an optical disc has been proposed on which an optically detectable guide groove is previously formed so as to record and play back information along the guide groove. Refer to, for example, Japanese Laid-open patent application No. 58-91536 (corresponding to U.S. Ser. No. 443,399). As to the optical disc of this type, a severe dimensional accuracy of the guide groove is required in order to secure the signal-to-noise ratio of a playback signal at a predetermined value.
There has also been proposed an apparatus which forms two laser beams and performs add-on writing by employing one of the laser beams for the tracking (playback) of recording tracks already bearing information (already recorded tracks) and the other for the recording. With this system, since the two laser beams are focused on a record carrier via individual optical systems, two beam spots are very difficult of stably holding a fixed distance on the record carrier, and the distance between them fluctuates due to any slight positional deviation of the individual optical systems. lt is, in effect, impossible to measure the distance between both the beam spots, and even the overlap of the two can occur due to a fluctuation in a track interval.